U Values
Remember this;
Heat energy flows from hot to cold
- the bigger the temperature difference the more heat energy flows in a
certain time (the power loss is bigger)
More heat will be lost from a bigger area
- double the area and the heat lost is doubled. They are proportional
Heat energy flows through some materials better than
others
- a thermal insulator has a small U value. They usually contain still air, e.g.
bubbles.
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When new homes are built they must meet very strict building
regulations.
One purpose of these is to ensure that the home will be well insulated.
There are a range of modern materials which can be used to insulate our
homes which have very low U values.
The U value of a material (or combination of materials) = the power loss
in watts though 1 m2 of the material for every degree centigrade
difference either side of the material.
P = U A
ΔT where P = power loss (W) , A = area (m2)
and ΔT is the temperature difference in 0C
e.g. a wall has a U value of 0.85 and an area of 10m2.
When the temperature difference across it is 120C
the the power loss = 0.85 x 10 x 12 = 102 W |
Remember ... Big U is bad - lots of heat loss Small U value is
good - little heat loss
Measure of a material's heat-conducting properties. It is used in the
building industry to compare the efficiency of insulating products, a good
insulator having a low U-value. The U-value of a material is defined as the rate
at which heat is conducted through it per unit surface area per unit temperature
difference between its two sides; it is measured in watts per square metre per
degree centigrade or Kelvin. (W/m2 K).
In mathematical terms, it may be calculated as the rate of loss of
heat/(surface area Ũ temperature difference).
| 50mm Polyisocyanurate |
0.340 W/mēK |
| 80mm Mineral Wool |
0.330 W/mēK |
| 60mm Phenolic |
0.291 W/mēK |
| solid brickwork |
2.033 W/mēK |
| brickwork with cavity |
1.382 W/mēK |
We can also calculate the U value for a whole house which is a combination of
all the surfaces and materials used.
Here are the maximum U values according to recent building regulations.
| Element |
maximum U-value
W/m2K |
| Walls |
0.30 |
| Floors |
0.22 |
| Flat roof or roof with integral insulation |
0.20 |
| Pitched roof (insulation at ceiling level) |
0.16 |
| Pitched roof (insulation at between rafters) |
0.20 |
| Windows, roof windows and roof lights |
1.80 |
| Doors with more than 50% of the internal surface area glazed |
2.20 |
| Other doors |
3.00 |